1. Field of the Invention
The present invention relates to a light-emitting diode (LED) driving circuit, and more particularly to a non-isolated LED driving circuit driving a switch device to control power supplied to the LEDs.
2. Description of the Related Art
Light-emitting diode (LED) is made of a compound including elements such as gallium (Ga), arsenic (As), phosphorus (P) and the like. Recombination of electrons and holes in the compound allows LED to radiate visible light. LED driving circuits have been widely used in products of consumer electronics and information technology, such as indoor and outdoor lighting, televisions and portable electronic devices. The LED driving circuits convert power supplied from an alternating current (AC) power source to direct current (DC) power. Additionally, LED driving circuits can also convert power from one voltage level to another voltage level.
The LED driving circuits can be classified into linear converters as well as switch-mode converters. The switch-mode converters are preferred over the linear converters because the switch-mode converters have higher efficiency than the linear converters, and, normally, power transistors, such as bipolar junction transistor (BJT), metal oxide semiconductor field effect transistors (MOSFET) and other types of transistors, are used as the power switches. The power switches generally receive pulse-frequency-modulated (PFM) and/or pulse-width-modulated (PWM) control signals from respective controllers.
With reference to FIG. 8, a conventional flyback LED driving circuit with a power switch includes a transformer 91, a controller unit 92, multiple LEDs 93, a power switch 94 and an electrolytic capacitor 95. The transformer 91 has a primary winding 911, a secondary winding 912 and an auxiliary winding 913. The power switch 94 is a field effect transistor, such as a high-voltage power MOSFET, and is used to control power delivered to the secondary winding 912 of the flyback LED driving circuit. For example, if the current of the primary winding 911 is greater than a threshold, the controller unit 92 turns off the power switch 94 and shuts down the flyback LED driving circuit.
As secondary-side feedback rules out the use of optocoupler and regulator in the LED driving circuit, the flyback LED driving circuit is thus complex in circuit design, making circuit miniaturization thereof hard to be achieved. Furthermore, instead of being directly used with a triac dimmer, the flyback LED driving circuit must have other circuits to function the same. Besides, the electrolytic capacitor 95 and the transformer 91 increase the cost and decrease the life time of the flyback LED driving circuit. Despite the LEDs 93 being a durable element, shorter life duration of the flyback LED driving circuit may arise from the electrolytic capacitor 95, which has a shorter life duration than other elements in the flyback LED driving circuit.